The present disclosure relates to methods of forming lithographic patterns, and particularly to methods of forming lithographic patterns employing bilayer metallic hard masks, and structures for effecting the same.
Single exposure lithography employing a 193 nm wavelength light source is approaching its limit at printing features having a pitch less than 80 nm. In order to print features having a pitch less than 80 nm, therefore, double exposure methods have been proposed.
Double via exposure methods as known in the art employ an organic planarization layer (OPL) to memorize two lithographic patterns, which are transferred into the organic planarization layer through two distinct combinations of lithographic exposure and development and a pattern transfer etch. During the second pattern trilayer lithography, the first pattern memorized in the lower OPL are filled with the upper OPL layer, and hence the first pattern needs to be re-opened during the second pattern transfer into the lower OPL. Since the OPL thickness to be removed in the first patterns includes both the lower OPL and the upper OPL, during the second pattern transfer into the lower OPL, the OPL etch time in the first patterns needs to be 100% longer than in the second pattern. For this reason, the fidelity of pattern transfer is compromised for the pattern that is transferred first. Specifically, the via holes derived from the second pattern tend to become wider and develop irregularities during the transfer of the first pattern into the organic planarization layer. Therefore, a method is desired to transfer two patterns with high fidelity to form a composite pattern including sublithographic features.